This invention relates to a process for producing an ultra-low phosphorus steel, which comprises applying oxygen top-blowing to a pig iron which has previously been desiliconized. More specifically, this invention relates to a process for producing an ultra-low phosphorus steel containing 0.010% or less of phosphorus by means of a top- and bottom-blowing steel refining process (hereunder "TB process").
It has long been desired in the art to reduce the phosphorus content of steel in an economical manner so as to further improve workability as well as mechanical properties of steel.
In the oxygen top-blowing steel making process, which is generally used in Japan, molten iron, scrap and other starting materials are charged into a converter and the refining of steel is carried out while blowing pure oxygen onto the charge melt through an oxygen lance. Usually, the phosphorus content of oxygen-refined steels is in the range of 0.015 to 0.035%.
An additional step is applied to such a steel making process as in the above so as to further lower the phosphorus content. Now considering the process for reducing the phosphorus content in steel, it is noted that there are the following three main practical methods in the art: (i) a double-slag process; (ii) pig iron dephosphorization; and (iii) dephosphorization after steel refining.
(i) The double-slag process is a two-stage refining process in which the first-stage refining is applied to molten steel in a relatively high carbon range, then refining is interrupted, and after separating the refined steel in a high carbon range from slag by tapping the molten steel or by removing the fluidized slag after the addition of fluorspar from the converter. The second stage refining is applied to the thus separated molten steel by adding another quicklime to the molten steel.
(ii) The pig iron dephosphorization, i.e., hot metal dephosphorization is a process in which a slag-forming flux containing quicklime, fluorspar and iron ore is introduced onto a pig iron bath in the ladle, while maintaining it at a sufficiently high temperature to effect dephosphorization by blowing an exothermic gas, such as oxygen into the bath. Alternatively, the dephosphorization can be carried out by adding a flux containing calcined soda or quicklime, fluorspar and iron ore to a molten pig iron which has been desiliconized to a level of 0.15% of Si. After dephosphorization, the thus desiliconized and dephosphorized pig iron is charged into a converter and a sufficient amount of quicklime is added to the bath so as to suppress the re-phosphorization in the converter. The addition of another large amount of quicklime is also effective in furthering the dephosphorization during steel refining.
(iii) The dephosphorization after steel refining is carried out by adding a flux containing quicklime, fluorspar and iron ore to the molten steel in the ladle or to the molten steel during tapping.
In such processes of steel refining, desiliconization has widely been applied as one of the means of pre-treatment of pig iron in order to reduce the requisite amount of quicklime, which is necessary as one of the auxiliary materials. The desiliconization is also effective in reducing the amount of slag which is formed during the refining process. It is, in fact, possible to reduce the requisite amount of quicklime by 16-17 kg per ton of pig iron when the proportion of silicon in pig iron is reduced to 0.13-0.16% by applying desiliconization to the pig iron prior to the refining. This is because some of the quicklime added is usually consumed to neutralize the SiO.sub.2 which is derived from the silicon dissolved in the pig iron during the oxygen refining process. Therefore, the amount of quicklime to be added is usually determined by considering the silicon concentration of the pig iron.
However, it is to be noted that the presence of silicon in pig iron is essentially necessary for steel refining, because the silicon in pig iron generates heat when it is oxidized during refining. The thus generated heat is effective in preparing slag, namely in melting the quicklime which is added to the bath as a slag-forming agent. Therefore, the reduction in silicon content in pig iron would result in less formation of slag.
On the other hand, the presence of quicklime in slag is necessary to dephosphorize a molten pig iron, since the molten quicklime in slag is combined with phosphorus in pig iron to achieve dephosphorization. Therefore, the presence of a substantial amount of quicklime in slag is essential for dephosphorization of pig iron during refining.
Therefore, though it is possible to apply desiliconization to a pig iron, it is not desirable to reduce the amount of quicklime to be added to the pig iron from the viewpoint of preparing an active slag for dephosphorization.
Thus, it has been thought in the art that it is impossible to apply desiliconization so as to produce low phosphorus steel and that any reduction in the phosphorus content requires a complicated and expensive process as long as the conventional dephosphorizing processes are concerned. In addition, such conventional dephosphorizing processes are always followed by a substantial reduction in tapping or total yield.
In this respect, U.S. Pat. No. 4,290,802 discloses the addition of a slag-forming agent such as quicklime to a molten steel in the TB process. However, it does not suggest anything about the employment of desiliconization as one of the means of pre-treating pig iron. Furthermore, the phosphorus content of steel which is produced in accordance with the process disclosed in this patent is 0.012% at the lowest.
It is herein to be noted that the degree of difficulty encountered in effecting dephosphorization depends on the starting phosphorus concentration. For example, it is not so difficult to reduce phosphorus from a level of 0.5% to a level of 0.05%. However, it is quite difficult to reduce the phosphorus content to 0.02% or less without reduction in tapping yield or without resulting in a prolonged period of treating time.
It has been thought that as long as the conventional process is concerned, it is impossible to achieve a CaO/SiO.sub.2 ratio of slag higher than 4. This is partly because the presence of much of the silicon is unavoidable, and partly because an amount of quicklime to make the ratio higher than 4 cannot be dissolved into the slag.